The invention relates to a cryostat having a connecting branch which is connected to a cooling chamber and is open on the end side, for example according to DE 39 24 579 A1.
Cryostats of this type are known and are used wherever an object has to be cooled to a very low temperature. Liquid nitrogen having a temperature of 77 K or liquid helium having a temperature of 4.3 K are usually used as coolant, which is provided in a coolant chamber of the cryostat. A cryostat is used, for example, in a magnetic resonance investigation device primarily used for medical purposes (cf., for example, DE 39 24 579 A1, EP 0 587 423 B1 or EP 0 736 778 B1), or else in investigation devices for analytical purposes in the chemistry field (cf., for example, U.S. Pat. No. 4,291,541 A). When a cryostat is used in a magnetic resonance tomograph, the cryostat is used for cooling the superconductive magnet used for generating the basic field. The cryostat in question has an open connecting branch, i.e. it is an open system in which the coolant chamber containing the liquid coolant is connected to the environment. The liquid coolant does not rapidly volatilize via the open connecting branch because a boiling equilibrium is set, and the supply of heat and energy to the coolant via the connecting branch is relatively small, so that very little coolant evaporates. Customary maintenance cycles within which coolant has to be topped up are approximately a year in the case of known magnetic resonance tomographs.
In the case of such a cryostat of a magnetic resonance tomograph, the connecting branch has a number of properties. Firstly, the first filling of the coolant chamber with coolant and topping up of the coolant can take place via the cryostat. Secondly, evaporating coolant can volatilize via the cryostat, the coolant having to volatilize in the case of an open system in order to avoid the internal pressure in the coolant chamber rising to an impermissibly high level. Moreover, the connecting branch is also used, if appropriate, for accommodating an electrode which is connected to the superconductive magnet when starting up the system. Via this electrode and a second electrode, which is likewise connected to the superconductive magnet, a current is guided over the superconductive magnet and, after reaching the transition temperature and with the magnet sufficiently cooled, is guided in a loss-free manner in the magnet, after which the two electrodes are separated from the superconductive magnet.
In this case, essentially three requirements have to be fulfilled by the connecting branch. Firstly, when accommodating an electrode it is to heat up as little as possible in order to avoid an impermissibly high transport of heat taking place in the direction of the coolant chamber via the connecting branch or via the shields insulating the coolant chamber to the outside. Furthermore, small transport of heat from the environment into the interior of the cryostat during operation is to take place via the connecting branch. Finally, a pressure loss which is as small as possible has to be provided when volatilizing coolant flows through the connecting branch, for example in the event of a quench. In the case of a quench, the superconductive magnet becomes impermissibly hot at one point and transfers into the standard conductive state, which is associated with local heating which spreads and results, in the worst case, in the entire superconductive magnet transferring into the standard conductive state. Above all, the transport of heat into the interior of the cryostat via the connecting branch has a great effect on the duration of the maintenance cycle. The lower the heat input, the longer can the maintenance cycles be, which has a significant effect on the competitiveness of the product.
In order to reduce the incorporation of heat by heat radiation, it is known to fit anti-radiation shields in the interior of the essentially cylindrical connecting branch, the shields being arranged in such a manner that the connecting branch is optically closed, as seen from the outside. That is to say, heat radiation can only be guided to the inside to a limited extent and is for the most part reflected by the anti-radiation shields. However, these anti-radiation shields result in a poorer quenching behavior, since, although the flow channel is open as before, they nevertheless form a sufficient flow resistance, which results in a relatively high pressure loss as the volatilized refrigerant flows through in the event of a quench.